The present invention relates to a method for growing a nitride semiconductor (InxAlyGa1xe2x88x92xxe2x88x92yN, 0xe2x89xa6X, 0xe2x89xa6Y, X+Yxe2x89xa61), and particularly to a method for growing a nitride semiconductor which can be used to make a nitride semiconductor substrate.
Recently various researches have been conducted on the growth of nitride semiconductor on a substrate made of a different material such as sapphire, spinel or silicon carbide which has a lattice constant different from that of the nitride semiconductor.
For example, a method of growing epitaxial lateral overgrowth GaN (ELOG) is described in JPN. J. Appl. Phys., vol. 37 (1998), pp. L309-L312, wherein nitride semiconductor having lower density of dislocations is obtained by forming a protective film of SiO2 or other material partially on a nitride semiconductor which has been grown on the C plane of sapphire, and growing nitride semiconductor thereon under a reduced pressure of 100 Torr.
In the ELOG growing process, nitride semiconductor having reduced dislocation defects can be formed on the protective film by intentionally growing the nitride semiconductor laterally on the protective film. When the nitride semiconductor grows, dislocation occurs and grows only in a window portion of the protective film.
However, in case the protective film of SiO2 or the like has wide stripe width, lateral growth of the nitride semiconductor on the protective film does not fully proceed eventually resulting in abnormal growth.
In addition, in case the nitride semiconductor is grown laterally by vapor phase deposition process, while two nitride semiconductor films which grow laterally from the nitride semiconductor exposed on both sides of the protective film meet and join with each other at the center of the protective film, dislocations concentrate locally at the joint. This is partly due to the fact that the front surface of the nitride semiconductor is tilted while growing laterally on the protective film of SiO2 or the like. In case a device layer is formed by epitaxial growth on a nitride semiconductor substrate such as the above, microscopic pits are likely to be generated in the joint where the dislocations are concentrated. The pits are generated by the dissociation of nitrogen in the process of heating the substrate for the purpose of growing the device layer. The pits grow larger as the epitaxial growth is continued.
As a result, even when a single continuous nitride semiconductor substrate is formed by growing nitride semiconductor layer laterally on a protective film by the vapor phase deposition process, it cannot be handled in the same way as an ordinary single crystal substrate. Since the active layer of a semiconductor laser should keep clear of the vicinity of the joint, it is difficult to secure a region large enough for forming the device. Moreover, since surface of the single nitride semiconductor substrate appears to be uniform, it has been difficult to recognize the joint by viewing the top surface of the substrate and to carry out the formation of device pattern accurately.
Furthermore, in case a single continuous nitride semiconductor substrate is formed by growing nitride semiconductor laterally by using a protective film on sapphire or the like, such a structure is likely to warp. Because sapphire, the protective film and the nitride semiconductor layer, which are stacked one on another, have different coefficients of thermal expansion.
The different-material substrate may also be removed from the nitride semiconductor substrate in the last stage. The substrate of different material maybe removed by polishing or irradiating the interface between the substrate and the nitride semiconductor with excimer laser thereby breaking the chemical bond in the interface. However, it has not been easy to remove a different-material substrate such as sapphire as it takes a long time to remove by polishing or by means of excimer laser.
An object of the present invention is to provide a new structure of nitride semiconductor substrate manufactured by lateral crystal growth with a protective film, which is capable of suppressing an adverse effect caused on the device by joining the nitride semiconductor layers on the protective film. Another object of the present invention is to prevent the nitride semiconductor substrate from warping. Still another object of the present invention is to facilitate removing a substrate made of a different material from the nitride semiconductor substrate.
In order to solve the problems described above, a nitride semiconductor substrate according to the first invention comprises (A) a supporting substrate, (B) a first nitride semiconductor layer having periodically arranged T-shaped cross section formed by laterally growing nitride semiconductor films starting at portions formed in a periodical stripe, grid or island configuration provided on the surface of the supporting substrate and stopping the lateral growth before the films join together, and (C) a second nitride semiconductor layer which is grown from the top surface or the top and side surface, which side surface has been grown laterally, of the first nitride semiconductor layer as the core and covers the entire surface of the supporting substrate, wherein cavities are formed under the joint of the second nitride semiconductor layer.
The nitride semiconductor substrate having such a structure as described above can be manufactured by (A) forming a protective film having windows of stripe, grid or island configuration on the supporting substrate, (B) laterally growing the first nitride semiconductor over the protective film from the exposed portions of the supporting substrate and stopping the growth in such a state as the protective film is not covered, (C) removing the protective film thereby to form cavities below the first nitride semiconductor layer which has been grown laterally, and (D) growing the second nitride semiconductor layer laterally from the top surface or the top and side surface, which side surface is the portion grown laterally, of the first nitride semiconductor layer. The supporting substrate may be either a substrate made of a different material such as sapphire or a different-material substrate covered with a nitride semiconductor layer over the entire surface thereof. In case a substrate made of sapphire or the like is used, it is preferable to form a low temperature-grown buffer layer on the substrate before growing the first nitride semiconductor. In case the second nitride semiconductor layer grows from the top surface of the first nitride semiconductor layer, the step of removing the protective film may be omitted since both parts of the second nitride semiconductor layer join with each other above the cavity even when the protective film is not removed.
According to the first aspect of the present invention, the nitride semiconductor without voids can be grown, even when forming the protective film widely. Also, strain can be suppressed which would otherwise be generated when the second nitride semiconductor is grown from the side surface of the first nitride semiconductor, because the second nitride semiconductor layer grows over the cavity. Moreover, since the front surface of the growing crystal does not tilt as in the case of growing on the protective film, concentration of dislocations in the joint can be relieved.
Also, it is made easier to locate the joint even from above the top surface of the second nitride semiconductor layer which covers the entire surface of the substrate, because such a cavity exists below the joint of the second nitride semiconductor layer that has a refractive index which is significantly different from that of the nitride semiconductor. Since the cavity relieves the strain, warping of the substrate due to the difference in the thermal expansion coefficient between the substrate and the nitride semiconductor layer can be mitigated.
Moreover, because the nitride semiconductor layer is supported by the discontinuous pillar-like structure on the supporting substrate, bonding strength between the nitride semiconductor layer and the supporting substrate decreases. As a result, not only the conventional method employing excimer laser, but also mechanical peeling technique such as vibration or thermal impact maybe used to remove the supporting substrate. The supporting substrate can be mechanically peeled off, for example, by polishing the supporting substrate on the back surface thereof and making use of the vibration generated during polishing. During polishing, the whole supporting substrate is peeled off by the vibration. When the mechanical peeling technique is employed, the supporting substrate can be removed in a shorter period of time. Although an interface where the peeling occurs tend to varies, uniform nitride semiconductor substrate can be obtained by polishing the supporting substrate on the back surface thereof after peeling.
When a different-material substrate covered by the nitride semiconductor layer is used as the supporting substrate, the nitride semiconductor layer covering the different-material substrate may be (a) a nitride semiconductor buffer layer grown at a temperature lower than the temperature at which the nitride semiconductor layer is to be grown thereafter (hereinafter called low temperature-grown buffer layer); (b) a laminate of a low temperature-grown buffer layer and a gallium nitride layer; (c) a laminate of a low temperature-grown buffer layer, a gallium nitride layer and an aluminum gallium nitride layer; or (d) a laminate of a low temperature-grown buffer layer, a gallium nitride layer and an indium gallium nitride layer.
Among the constitutions described above, use of nitride semiconductor layer (c) (=the laminate of low temperature-grown buffer layer, gallium nitride layer and aluminum gallium nitride layer) has an effect of suppressing a decomposition of the nitride semiconductor layer on the supporting substrate surface in the subsequent process thereby to prevent the generation of V-shaped grooves which would otherwise be generated in the supporting substrate surface. It is also made easier to peel off the supporting substrate by utilizing the stress generated by the difference in thermal expansion coefficient between the gallium nitride layer and the aluminum gallium nitride layer. When nitride semiconductor layer (d) (=the laminate of a low temperature-grown buffer layer, a gallium nitride layer and an indium gallium nitride layer) is used, peeling off the supporting substrate is made easier by utilizing the fact that the indium gallium nitride layer has mechanical strength weaker than that of the gallium nitride.
Windows of stripe, grid or island configuration are formed on the protective film on the supporting substrate. It is preferable to form windows of grid or island configuration. When the windows of grid or island configuration are formed, the first nitride semiconductor layer grows in many directions in the plane thus making it easier to peel off the supporting substrate. It is more preferable to form the windows of grid configuration so that the protective film surrounded by the window has polygonal or circular shape. When the area of the protective film surrounded by the window is formed in polygonal or circular shape, the joint of the second nitride semiconductor layer becomes a point at the center of the protective film thus making it possible to minimize the area of the joint where dislocations are concentrated.
While the protective film is removed after growing the first nitride semiconductor layer, the protective film may not be completely removed and it suffices to remove the protective film in such a manner as at least a cavity is formed under the joint of the second nitride semiconductor layer. For example, the protective film may be removed only from under the joint, or just be reduced its thickness.
The protective film may be removed by dry etching or wet etching, in either way the protective film can be removed without degrading the crystallinity of the nitride semiconductor. Dry etching is capable of easily controlling the depth of the protective film to be removed.
When the protective film is removed so as to expose the surface of the supporting substrate, the problems caused by the decomposition of the protective film while growing the nitride semiconductor on the protective film, namely abnormal growth and degradation of crystallinity of the nitride semiconductor can be mitigated.
The protective film is made of silicon oxide, silicon nitride, titanium oxide or zirconium oxide, or a multi-layered film of these materials or a film made of a metal having a high melting point not lower than 1200xc2x0 C. Since such a material for the protective film has the property of not allowing nitride semiconductor to grow easily thereon, the protective film is preferably used for growing the nitride semiconductor laterally thereon.
The nitride semiconductor substrate according to the second invention has a nitride semiconductor layer which is grown laterally starting at portions of the substrate formed in a periodical stripe or grid configuration provided on the surface of the supporting substrate, wherein the two films of the nitride semiconductor layer grown from the respective starting points do not join with each other but oppose each other via a clearance.
Thus the nitride semiconductor substrate of the present invention is characterized by the configuration of the two films of the nitride semiconductor layer, grown from the respective starting points, not joining with each other but disposed to oppose each other via a clearance, on the contrary to the conventional wisdom of the laterally grown substrate. We found that, even in the case of the nitride semiconductor substrate whereon the two films of the nitride semiconductor layer grown laterally are disposed to oppose each other via a clearance, crystal to make such a device as laser or LED can be grown flatly by the vapor phase epitaxial process. We also found that, since the epitaxial growth is started in such a state as there is no joint where dislocations would be concentrated, there occurs no generation of pits due to the dissociation of nitrogen when the substrate is heated which has been a problem in the prior art. Further, it is made possible to grow a flat device layer having better crystallinity than those obtained by the prior art.
The nitride semiconductor substrate having such a structure as described above can be manufactured by, for example, forming a protective film having stripe, grid or island-like configuration on the supporting substrate, laterally growing the nitride semiconductor over the protective film from the exposed portions of the supporting substrate and stopping the growth in such a state as the protective film is not completely covered. The supporting substrate maybe either a substrate made of a different material such as sapphire or a different-material substrate covered with a nitride semiconductor layer over the entire surface thereof.
It is preferable to form cavities by removing the protective film under the laterally-grown nitride semiconductor. Forming the cavities makes it easier to locate the clearance in the subsequent device forming process. Also the strain generated by difference of the coefficients of thermal expansion between the different-material substrate and the nitride semiconductor can be mitigated, thereby to suppress the warping of the nitride semiconductor substrate. Preferable structure and composition of the supporting substrate, material and shape of the protective film and method of removing the protective film are similar to those of the first investment.
The nitride semiconductor substrate of the present invention may comprises (A) the nitride semiconductor layer having low density of dislocations obtained according to the first invention or the second invention, (B) a thick nitride semiconductor layer for dispersing dislocations of the nitride semiconductor layer which has grown by halide vapor phase epitaxy process (hereinafter called the HVPE process), and (C) a nitride semiconductor layer formed by the similar way as the first invention or the second invention. In the nitride semiconductor layer obtained according to the first invention or the second invention, dislocations remain above the windows of the protective film. By dispersing the dislocations by means of the thick nitride semiconductor layer formed by the HVPE growing process, the nitride semiconductor layer can be made to have a relatively low dislocation density over the whole area. By growing the layers according to the first invention or the second invention on the base of the HVPE grown nitride semiconductor layer, a nitride semiconductor substrate having even lower density of dislocations can be obtained.